I just tried this with Garmin maps, City Navigator Europe V9, in the MapSource program (i.e, on your PC).
It worked fine from southern France to a small town in Belgium, and then on to central Scotland. It selected the Channel Tunnel and looks like it routed through the parking lots where you'd do the roll-on roll-off routines.
One of the routing preferences I have set under 'Try to avoid' is Ferries. I suspect it must have based the choice (the multiple ferries vs. the Chunnel) on the travel times.